In the field of graphic arts, an image formation system capable of giving a high contrast photographic characteristic is required to attain a good reproduction of a halftone image of continuous gradation as well as a good reproduction of a line image.
A specific developer called a lith developer has heretofore been used for said purpose. The lith developer contains only hydroquinone as a developing agent, and uses a sulfite preservative in the form of an adduct of sulfite with formaldehyde in order not to deteriorate the infectious development, therefore the concentration of free sulfide ion in said developer must be kept extremely low. Thus, the lith developer is extremely easily oxidized with air and can not last more than three days, which is a serious defect.
Methods for obtaining a photographic characteristic of high contrast by the use of a stable developer are disclosed in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857 and 4,243,739, where a hydrazine derivative is used. According to said methods, a photographic characteristic of high contrast and high sensitivity may be obtained, and moreover, addition of a sulfite of high concentration to a developer is possible. Accordingly, the stability of the developer against aerial oxidation is markedly improved, as contrasted with the conventional lith developer.
In said methods using a hydrazine derivative, however, the pH value of the developer is kept higher than that of the conventional lith developer, and therefore, said pH value of the developer is apt to easily vary, resulting in a variation of the developed photographic characteristics. This is a troublesome problem.
U.S. Pat. No. 4,269,929 discloses, in order to solve said problem, the use of an alkaline developer containing a dihydroxybenzene developing agent and a 3-pyrazolidone developing agent, to which an amino compound is added so as to improve the activity of the developer; whereby said developer having a lower pH value can show the high sensitivity and high contrast image forming effect of the hydrazine derivative.
It is impossible, however, to sufficiently lower the pH value of the developer enough to stop the variation of said pH value under the condition of general preservation of the developer or conventional use of the developer in a developing machine (i.e., without severe control of a temperature, pH, amount and the like of the developer) even by said means as described in U.S. Pat. No. 4,269,929.
In addition, the amino compounds added to said developer act as a solvent of a silver halide. (Refer to The Theory of the Photographic Process, 3rd Ed., p. 370, written by C. E. K. Mees, and Photographic Processing Chemistry, p. 43, written by L. F. A. Mason.) Under the circumstances, the method for the development according to said U.S. Pat. No. 4,269,929 where a large amount of amino compounds is used has the problem of a so-called "silver stain". Said "silver stain" is a defective phenomenon, often occurring during development. Silver stain occurs, for example, as follows: In a method for development of a silver halide photographic material using an automatic developing machine where a replenishing solution is fed into the development tank, in accordance with the area of the film to be developed, the developer in the tank is used for a long period of time. As a result, the silver halide component released from the film is deposited and precipitated on the surface of the wall of the development tank or on the surface of the roller of the film conveyor. This silver deposit is then transferred to new film that is developed. This silver deposit transferring to new film is a defect, and is the silver stain in question.
A general means to eliminate said defective silver stain is to add some compounds, which are called silver stain inhibiting agents, to the developer. Various kinds of compounds have heretofore been known as said silver stain inhibiting agents and include, for example, 2-mercapto-1,3,4-thiadiazoles (British Pat. No. 940,169), 2-mercapto-1,3,4-oxadiazoles or 1-phenyl-5-mercaptotetrazoles (U.S. Pat. No. 3,173,789), DL-6,8-dithiooctanoic acid (U.S. Pat. No. 3,318,708), o-mercaptobenzoic acid (British Pat. No. 1,144,481), aliphatic mercaptocarboxylic acids (U.S. Pat. No. 3,628,955), L-thiazoline-4-carboxylic acid (J. Photogr. Sci., 13, 233 (1965)), 2-mercaptobenzoxazole or 2-mercaptobenzimidazole (Photogr. Sci. Eng., 13, 220 (1976), Japanese Patent Application (OPI) Nos. 24347/81 and 72411/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")), etc.
It has been noticed, however, that many of said silver stain inhibitory agents are, when applied to a high contrast development process using a hydrazine derivative, apt to injure or deteriorate the effect of sensitization and high contrast development attainable by said hydrazine derivative. Under the circumstances, some other compounds capable of effectively preventing the silver stain without deteriorating the effect of the hydrazine derivatives for sensitization and high contrast development must be used.
No compounds have heretofore been known, which may satisfy said requirement.